On 11 Aug 2015 at 17:07, G. Lofstead wrote:
> I still have my original Cetron tubes in my SB200.
> I built it when the SB200 first come out in the 60's...
>> Never abused and never ran high grid current, only
> enough to make the power i wanted.
>> Still going strong.. 8-)
Yes. You're a very wise man, Jerry.
First of all GRID current is the parameter to watch. Adjust everything for peak
GRID current, while keeping it "low" by adjusting the drive level.
Although the original Cetrons were rated to accept grid currents of 50 mA or
more per tube in the SB-200, in my experience that is completely
unnecessary.
I have heard repeatedly that some SB-200 owners brag about achieveing 750
watts output, in point of fact, one really shouldn't drive the SB-200 to more
than 600 watts output in CW. That extra 150 watts can just barely be noticed
on the receiving end, so why bother?
And the SB-200 does not need 100 watts of drive to achieve this 600 watt
output either.
Both of mine require something around 50 watts. In fact, I can drive either to
600 watts output on 80 meters with a HW-16.
**I** adjust everything for peak GRID current, adjusting drive to keep the grid
current well under 50 mA per tube, usually, in fact, to about 35 to 40 mA per
tube, as I said above.
This is one reason I really don't like the FL-2100 series of Yeasu SB-200
copies: they have no means of measuring grid current.
Grid current, in the SB-200, if not in all grounded-grid linear amps, at least
those using 572Bs, is a FAR more important parameter to keep track of than
plate current.
I let the plate current take care of itself, only occasionally looking at it to make
sure I am not letting it get above 500 mA in my SB-200. It very seldom is,
unless the amp is out of resonance.
After everything is adjusted for peak grid current at the level of drive that
keeps grid current to the value I prefer, 35 - 40 mA or less, I then adjust the
"LOADING" control "forward" to cause the plate current to drop a couple of
mils. This (supposedly) enhances linearity by more heavily loading the plate
circuit.
Again, YMMV.
Ken Gordon W7EKB